His mother's brethren, i.e. kinsmen, as that word is oft used, as Genesis 14:16, Genesis 29:12. He is our brother; they were easily persuaded to believe and follow what served their own interest.
Related Commentaries of Judges 9:3
Judges 9:3
3 And his mother's brethren spake of him in the ears of all the men of Shechem all these words: and their hearts inclined to followb Abimelech; for they said, He is our brother.